Method for Recovering Gold, Silver, Copper and Iron from Plasma-Caused Slag Containing Valuable Metals

ABSTRACT

There is disclosed an environmentally friendly method for recovering gold, silver, copper and iron from valuable metal-contained plasma-molten slag. At first, plasma is used to burn the used printed circuit boards, thus producing the slag. Then, the slag is grinded. Then, leaching, crystallization, precipitation, replacement and electric winning are conducted to recover gold, silver, copper and iron.

FIELD OF THE INVENTION

The present invention relates to an environmentally friendly method forrecovering gold, silver, copper and iron from valuable metal containedplasma-molten slag via plasma burning, grinding, leaching,crystallization, precipitation, replacement and electric winning.

DESCRIPTION OF THE RELATED ARTS

Used printed circuit boards contain valuable metals such as silver,gold, copper and iron. If they are disposed of without recovering thevaluable metals contained, it will be a hazard to the environment andloss of resources. Conventionally, the used printed circuit boards canbe directly crushed before the valuable metals are recovered asdisclosed in Taiwanese Patent Publication Nos. 247281 and 36904 forexample. However, it consumes excessive energy because they contain alot of elastic resin that is difficultly crushed to debris. Moreover,there are problems related to production of undesired dust and noise andwearing of machines.

Alternatively, the conventional burning method can be used forpretreatment. However, it takes much time and therefore expenses a lotof energy to burn the used printed circuit boards for low combustionefficiency.

As disclosed in Taiwanese Patent Publication No. 1268184, the usedprinted circuit boards are heated to a temperature higher than 200degrees Celsius to melt solder on the used printed circuit boards sothat electronic parts can be removed from the used printed circuitboards. Then, the electronic parts are submerged in a solvent anddissolved so that the valuable metals can be recovered.

Therefore, the present invention is intended to obviate or at leastalleviate the problems encountered in prior art.

SUMMARY OF THE INVENTION

It is the primary objective of the present invention to provide anenvironmentally friendly method for recovering valuable metals from usedprinted circuit boards.

To achieve the foregoing objective, in the method, plasma is used toburn the used printed circuit boards, thus providing slag. The slag isgrinded to debris smaller than 2 mm. A sieve with meshes of 0.149 mm isused to screen the debris into two fractions, the larger and the smallerones. A magnet is used to separate ferromagnetic debris fromnon-ferromagnetic debris so that the ferromagnetic debris can beprovided to a steel-making factory. The non-ferromagnetic large debris,which is rich in copper, can be provided to a copper refinery. Thenon-ferromagnetic small debris, which still contains valuable metals,can be further treated for recovering gold, silver and copper. 18Nsulfuric acid is used to leach the debris. The solid/liquid ratio isretained at 10 g/50 ml, and the operational temperature is kept at 70degrees Celsius so that 90.56% of the copper is released from thenon-ferromagnetic small debris to the sulfuric acid after 1 hour. Thecopper-contained primary leaching solution is subsequently separatedfrom the primary sulfuric acid-leached residue and retained at 27degrees Celsius for 48 hours for crystallization. 58.28% of the copperis precipitated and recovered in the form of copper sulfate crystals.The copper remained in the crystallization filtrate is replaced withiron powder, which is used as replacement reagent. 100% of the coppercan be recovered as copper powder from the filtrate when iron is addedat 100 times of the theoretical amount. 18N sulfuric acid is used toleach the primary sulfuric acid-leached residue again. The solid/liquidratio is retained at 50 g/50 ml, and the temperature is kept at 70degrees Celsius so that 100% of the copper is released from the primarysulfuric acid-leached residue to the sulfuric acid after 2 hours. Thesecondary copper-contained leaching solution is then separated from thesecondary sulfuric acid-leached residue. The copper in the solution isagain replaced with iron powder so that copper is completely recoveredwhen iron is added at 150 times of the theoretical amount. 8N nitricacid is used to leach the secondary sulfuric acid-leached residue,wherein the solid/liquid ratio is retained at 1 g/50 ml, and thetemperature is kept at 70 degrees Celsius so that 100% of the silver isreleased to the nitric acid from the residue after 4 hours. Then theoptimal silver-contained leaching solution is separated from the nitricacid-leached residue and using ammonia solution to adjust the pH valueto 10. Subsequently 12N hydrochloric acid is provided for precipitationreaction. The ratio of the hydrochloric acid to the silver-containingleaching solution is 1:4 so that 100% of the silver is recovered in theform of silver chloride. The nitric acid-leached residue is then treatedwith 100% aqua liquid for recovering gold. The ratio of the residue tothe aqua liquid is 0.5 g/50 ml, and the temperature is kept at 70degrees Celsius so that 100% of the gold is released to the aqua liquidfrom the nitric acid-leached residue after 4 hours, thus providingoptimal gold-contained leaching solution. The solution is treated withzinc powder, as the replacement reagent, to recover about 99.43% of thegold from the optimal gold-contained leaching solution.

Other objectives, advantages and features of the present invention willbecome apparent from the following description referring to the attacheddrawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The present invention will be described via detailed illustration of thepreferred embodiment referring to the drawing.

FIG. 1 is a flow chart of a method for recovering valuable metals fromvaluable metal-contained plasma-molten slag according to the preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, it reveals a method for recovering valuable metalsfrom valuable metal-contained plasma-caused slag according to thepreferred embodiment of the present invention. At 1, the slag iscollected.

At 2, the slag is screened with a sieve with meshes of 0.149 mm. Thus,large debris 21 larger than 0.149 mm in size is separated from smalldebris 22 smaller than 0.149 mm in size.

At 3, the large debris 21 is tested with a magnet so that ferromagneticdebris 31 is separated from non-ferromagnetic debris 32. Theferromagnetic debris 31 can be provided to a steel-making factory. Thenon-ferromagnetic debris 32 is rich in copper and can be provided to acopper refinery.

At 4, the small debris 22 is tested with a magnet so that ferromagneticdebris 41 is separated from non-ferromagnetic debris 42. Theferromagnetic debris 41 can be provided to a steel-making factory.

At 5, primary sulfuric acid leaching occurs. The non-ferromagneticdebris 42 is leached with 18N sulfuric acid. During the leaching, thesolid/liquid ratio is retained at 10 g/50 ml, and the temperature iskept at 70 degrees Celsius. After 1 hour of leaching, about 90.56% ofthe copper is released from the non-ferromagnetic debris 42 to thesulfuric acid. Primary copper-contained leaching solution 51 isseparated from primary sulfuric acid-leached residue 52.

At 6, the copper-contained leaching solution 51 is retained at 27degrees Celsius for 48 hours for crystallization. Thus, 58.28% of thecopper is recovered in the form of copper sulfate crystals 61. Thecopper sulfate crystals 61 are separated from optimal crystallizationfiltrate 62.

At 7, the copper in the optimal crystallization filtrate 62 is replacedwith iron powder that is used as replacement reagent 7. When the amountof the iron reaches 100 times of the theoretical value, 100% of thecopper is recovered from the optical crystallization filtrate 62. Copperpowder 71 is recovered.

At 8, secondary sulfuric acid leaching occurs. The sulfuric acid-leachedresidue 52 is leached with 18N sulfuric acid again. During the leaching,the solid/liquid ratio is retained at 50 g/50 ml, and the temperature iskept at 70 degrees Celsius. After 2 hours of leaching, about 100% of thecopper is released from the sulfuric acid-leached residue 52 to thesulfuric acid. Then, secondary copper-contained leaching solution 81 isseparated from secondary sulfuric acid-leached residue 82.

At 9, the copper in the secondary copper-contained leaching solution 81is replaced with iron powder that is used as replacement reagent 9. Whenthe amount of the iron reaches 150 times of the theoretical value, 100%of the copper is recovered from the secondary copper-contained leachingsolution 81. Copper powder 91 is recovered.

At 10, the sulfuric acid-leached residue 82 is leached with 8N nitricacid. During the leaching, the solid/liquid ratio is retained at 1 g/50ml, and the temperature is kept at 70 degrees Celsius. After 4 hours ofleaching, about 100% of the silver is released to the nitric acid fromthe sulfuric acid-leached residue 82. Optimal silver-contained leachingsolution 101 is separated from nitric acid-leached residue 102.

At 11, the pH of the optimal silver-contained leaching solution 101 isadjusted with ammonia solution.

At 12, 12N hydrochloric acid is used as precipitating agent. During theprecipitation, the ratio of the hydrochloric acid to thesilver-contained leaching solution 101 is 1:4. Thus, 100% of the silveris recovered in the form of silver chloride 121.

At 13, the nitric acid-leached residue 102 is leached with 100% aqualiquid. During the leaching, the ratio of the nitric acid-leachedresidue 102 to the 100% aqua liquid is 0.5 g/50 ml. The temperature is70 degrees Celsius. After 4 hours, 100% of the gold is released to theaqua liquid from the nitric acid-leached residue 102, thus providingoptimal gold-contained leaching solution 131.

At 14, the gold in the optimal gold-contained leaching solution 131 isreplaced with zinc powder that is used as replacement agent. Thus,99.43% of the gold is recovered from the optimal gold-contained leachingsolution 131. Gold 141 is collected.

The method of the present invention exhibits several advantages.Firstly, gold, silver, copper and iron are rapidly recovered from usedprinted circuit boards. Therefore, the valuable metals can be processedin refineries and reused to reduce the waste of metals. Secondly,hazardous materials produced during the recovering of the valuablemetals are reduced. Therefore, the hazard to the environment and humanbodies is reduced.

The present invention has been described via the detailed illustrationof the preferred embodiment. Those skilled in the art can derivevariations from the preferred embodiment without departing from thescope of the present invention. Therefore, the preferred embodimentshall not limit the scope of the present invention defined in theclaims.

The present invention has been described via the detailed illustrationof the preferred embodiment. Those skilled in the art can derivevariations from the preferred embodiment without departing from thescope of the present invention. Therefore, the preferred embodimentshall not limit the scope of the present invention defined in theclaims.

1. A method for recovering valuable metals from used printed circuitboards comprising the steps of: providing plasma for burning the usedprinted circuit boards, thus providing slag; grinding the slag;providing a sieve with meshes of 0.149 mm for sieving the slag, thusseparating large debris larger than 0.149 mm from small debris smallerthan 0.149 mm; providing a magnet for testing the large debris, thusseparating ferromagnetic large debris from non-ferromagnetic largedebris so that the ferromagnetic large debris can be provided to asteel-making factory while the non-ferromagnetic large debris, which isrich in copper, can be provided to a copper refinery; providing a magnetto test the small debris, thus separating ferromagnetic small debrisfrom non-ferromagnetic small debris so that the ferromagnetic smalldebris can be provided to a steel-making factory; providing 18N sulfuricacid for leaching the non-ferromagnetic small debris, wherein thesolid/liquid ratio is retained at 10 g/50 ml, and the temperature iskept at 70 degrees Celsius so that 90.56% of the copper is released fromthe non-ferromagnetic small debris to the sulfuric acid after 1 hour,thus separating primary copper-contained leaching solution from primarysulfuric acid-leached residue; retaining the copper-containing leachingsolution at 27 degrees Celsius for 48 hours for crystallization so that58.28% of the copper is recovered in the form of copper sulfatecrystals, thus separating the copper sulfate crystals from optimalcrystallization filtrate; replacing the copper in the optimalcrystallization filtrate with iron powder used as replacement reagent sothat 100% of the copper is recovered from the optical crystallizationfiltrate when the amount of the iron reaches 100 times of thetheoretical value, thus providing copper powder; providing 18N sulfuricacid for leaching the sulfuric acid-leached residue again, wherein thesolid/liquid ratio is retained at 50 g/50 ml, and the temperature iskept at 70 degrees Celsius so that 100% of the copper is released fromthe sulfuric acid-leached residue to the sulfuric acid after 2 hours,thus separating secondary copper-containing leaching solution fromsecondary sulfuric acid-leached residue; replacing the copper in thesecondary copper-contained leaching solution with iron powder used asreplacement reagent so that 100% of the copper is recovered from thesecondary copper-contained leaching solution when the amount of the ironreaches 150 times of the theoretical value, thus providing copperpowder; providing 8N nitric acid for leaching the sulfuric acid-leachedresidue, wherein the solid/liquid ratio is retained at 1 g/50 ml, andthe temperature is kept at 70 degrees Celsius so that 100% of the silveris released to the nitric acid from the sulfuric acid-leached residueafter 4 hours, thus separating optimal silver-containing leachingsolution from nitric acid-leached residue; providing ammonia solutionfor adjusting the pH of the optimal silver-contained leaching solution;providing 12N hydrochloric acid for precipitation, wherein the ratio ofthe hydrochloric acid to the silver-contained leaching solution is 1:4so that 100% of the silver is recovered in the form of silver chloride;providing 100% aqua liquid for leaching the nitric acid-leached residue,wherein the ratio of the nitric acid-leached residue to the 100% aqualiquid is 0.5 g/50 ml, and the temperature is 70 degrees Celsius so that100% of the gold is released to the aqua liquid from the nitricacid-leached residue after 4 hours, thus providing optimalgold-contained leaching solution; and replacing the gold in the optimalgold-contained leaching solution with zinc powder used as replacementagent, thus recovering 99.43% of the gold from the optimalgold-contained leaching solution.